Lyme borreliosis is now the most common arthropod-borne disease in the United States. Borrelia burgdorferi, the causative agent, has been isolated from humans, mammals, birds and arthropods and is cultivable in the laboratory. B. burgdorferi has an unusual genome composed of a linear chromosome of about 950 kb and variable numbers of linear and circular plasmids that range in size from about 5 to 60 kb. The linear and circular plasmids have copy numbers of about one per chromosome. Some of these plasmids are lost during in vitro cultivation. The linear plasmids have covalently closed hairpin ends with structures and sequences similar to those of African swine fever virus telomeres. Only four genes have been mapped currently to plasmids; the remainder are on the chromosome. The genes for the major outer-surface proteins OspA and OspB are on a 49 kb linear plasmid in B31 and on a slightly larger plasmid in strains ACAI and R-IP90. The gene for OspD is on a 38 kb linear plasmid in B31 that is often lost upon in vitro cultivation. The gene for OspC, a major immunodominant protein, is on a 26 kb circular plasmid in all three Lyme disease species and therefore represents the first Borrelia gene mapped to a circular DNA molecule. The objective of this project, therefore, is to define the genetic capacity of B. burgdorferi and other vector-borne pathogens in sufficient detail to begin the process of mapping those genes and gene products which may be important in pathogenesis.